1. Field of the Invention
The present invention relates generally to improvements in mechanical transmissions and, more particularly, but not by way of limitation, to improvements in continuously variable transmissions that provide a range of output speeds for a fixed input speed.
2. Description of the Prior Art
It has long been known that internal combustion engines will operate most efficiently at a fixed speed which is determined by the characteristics of the particular engine. Thus, considerable savings in energy could be achieved by operating, for example, an automobile engine at constant speed while driving the wheels of the automobile at varying speeds through a transmission that is capable of providing a continuous range of speeds of an output shaft of the transmission for a fixed speed at which an input shaft of the transmission is driven. Because of this possibility of achieving energy savings, continuously variable transmissions have undergone considerable development in recent years.
The common approach that has been taken with prior art continuously variable transmissions has been to provide the transmission with two split sheaves connected together by a belt. One sheave is mounted on an input shaft and the other sheave is mounted on an output shaft for the transmission. The splitting of the sheaves, which is effected on a plane perpendicular to the shaft upon which each sheave is mounted, permits the sides of each sheave to be separated by greater or lesser distances so that the belt that is connecting the sheaves can be caused to engage the sheaves at varying distances from their centers. By coordinately adjusting the separation of the sides of both sheaves, a continuous range of output shaft speed to input shaft speed can be effected.
While prior art transmissions based on this approach have proven useful for many applications, they suffer from several drawbacks. In general, since the two sheaves are connected by a belt, such a transmission can be bulky so that it is not suitable for some applications. Similarly, the adjustment of the speed ratio of the two shafts requires coordination of the adjustment of the separation of halves of sheaves while the sheaves are rotating and, further, the coordination of the adjustments to these sheaves-half separations must be relatively precise if slippage of the belt is not to occur. The mechanisms required to make such precise adjustments to moving parts with the requisite degree of coordination of the adjustments can be relatively expensive.